An integrated circuit (IC) may include a constituent section that operates at a voltage different from another constituent section. Interfacing between constituent sections operating at different voltages may require a buffer circuit that may include active elements (e.g., Metal-Oxide-Semiconductor (MOS) transistors) operating at a voltage (e.g., 1.8 V) lower than a voltage across terminals thereof.
FIG. 1 shows a schematic view of an output stage 100 of a buffer circuit. The output stage 100 may include a p-channel Metal-Oxide-Semiconductor (PMOS) transistor M1 102 and an n-channel MOS (NMOS) transistor M2 104. The source (S) terminal of M1 102 may be connected to a supply voltage VDDIO 106, and the source (S) terminal of M2 104 may be connected to a supply voltage VSS 110. The bulk (B) terminals of the transistors (M1 102, M2 104) may be shorted with the source (S) terminals thereof to connect the bulk (B) terminals of the transistors (M1 102, M2 104) also to VDDIO 106 and VSS 110 respectively. The drain (D) terminals of M1 102 and M2 104 may be connected to one another, as shown in FIG. 1.
An external voltage from an Input/Output (IO) pad 108 of an IC may be supplied to each of the drain (D) terminals of M1 102 and M2 104. The gate (G) terminals of the transistors (M1 102, M2 104) may be driven by control signals (CTRL1 112 and CTRL2 114) generated from a control circuit of the buffer circuit. When the IO pad 108 voltage (e.g., 3.465 V) is higher than the supply voltage, VDDIO 106 (e.g., 1.8 V, 2.5 V), the parasitic diode D1 116, shown in FIG. 1 as being associated with M1 102, may be turned ON, leading to there being a direct path between the IO pad 108 voltage and the supply voltage VDDIO 106. The turning ON of D1 116 may lead to the conducting of a large current, which, in turn, may cause large leakage currents to flow. FIG. 1 also shows the parasitic diode D2 118 associated with Q2 104.
A high value of the IO pad 108 voltage may, therefore, compromise the reliability of the buffer circuit.